1. Field of the Invention
This invention relates generally to small computer system interfaces (SCSI), and more particularly to methods and apparatus for terminating a SCSI bus.
2. Statement of the Prior Art
Intelligent drive arrays are well known. For example, the COMPAQ SYSTEMPRO Personal Computer System which is manufactured by the assignee of the present invention offers the flexibility to be configured to meet a wide range of user needs, and is available in several standard model configurations each of which includes an intelligent drive array controller that is adaptable to two-drive, four-drive, six-drive and eight-drive arrays. Further information regarding a COMPAQ SYSTEMPRO Personal Computer System can be found in Technical Reference Guide: COMPAQ SYSTEMPRO Personal Computer System. Volumes I and II, Compaq Computer Corporation (1989).
The drive arrays which are used in such personal computer systems often employ a SCSI. As should be well known to those of ordinary skill in the art, SCSI's comprise a byte-wide data bus with an optional parity bit and nine signals to control data transfers between peripheral devices and a host system, among multiple peripheral devices, and among multiple host systems. Each host system has a host adaptor (i.e., a device which couples the host system to the SCSI bus and performs lower levels of SCSI protocol) which normally acts as an "initiator", to request an I/O process to be performed by another SCSI device known as a "target". Targets may comprise other host adapters or controllers that couple the peripheral devices to the SCSI bus.
SCSI devices (whether they are initiators or targets) are daisy-chained together, typically with a common, 50-conductor "A" cable. Alternatively, they may be daisy-chained together using a 68-conductor "B" cable. Both ends of either cable are terminated for two basic reasons: (1) to prevent noise by maintaining a high signal-to-noise ratio and (2) to maintain the SCSI bus in a known state. Further information regarding termination may be found in the American National Standard "Small Computer System Interface", ANSI X3.131-1986 or the draft proposed American National Standard "Small Computer System Interface", ANSI X3.131-198X.
The simplest prior art approach for terminating a SCSI bus has been to plug a cable adaptor containing terminating resistors for each wire into the unused port of the end device. When the end device is removed from the chain or moved to a different place on the chain, then the cable adaptor must be removed from the moved device and connected to the unused port of the end device.
The second approach has been to include a removable package of terminating resistors in a socket on the printed circuit (PC) board of the device. By removing an access panel on the device, the package can be reached and manually pulled off or pushed into the socket on the PC board depending on the location of the device on the chain.
These two approaches have the drawback that the resistors are in a separate unit which must be manipulated and stored independently of the device. In addition, the cable adaptor is generally packaged, sold and shipped separately from the devices which greatly increases its cost. The removable resistor package requires the device user to open up an access panel on the device housing, locate the terminating resistors and replace them at the last device on the chain. See also, for example, U.S. Pat. No. 4,920,339.